Teachers are often frustrated by the difficulty finding legally appropriate resources for school projects. What's OK to use, what's not? Two areas where you can feel confident using the resources are when they're in the Public Domain or when they are covered by a Creative Commons license:
A comprehensive list of places where the resources are all covered by Public Domain or Creative Commons:
Wikimedia Commons has nearly 6,000,000 images that you can freely use:
Best Practices
This is a great resource if you're looking for what teachers can and can't do legally:
Royalty Free Isn't Free
Many teachers think that so long as the content says Royalty Free, they're good to go. This is not the case. Royalty Free means that once you license use (almost always for money), you can use the media over and over again without paying a royalty (fee) for each usage. This kind of license is typically cheaper than a rights-managed license, but the rights-managed license is usually more exclusive (so it's less likely your competitor is less likely to be using the same piece of media). For a more detailed discussion see this:
More Possibilities
You may also find more here. Be sure to read about licensing to be sure:
This is a pretty slick web site to give you a little more information in a fairly digestible format. Read instructions on the site:
Copyright Info from California Media Festival
Other
Copyright Friendly Resources